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How does an aircraft derive winds aloft on its own?
The G1000 has a feature where it can put up a winds aloft arrow/speed in the PFD.  I've been sort of wondering how exactly this is determined.  My guess is that the aircraft (or the G1000) knows your airspeed via the pitot/static system and heading via the fluxgate/mag compass.  Then, it uses your GPS position to figure out where you should be every X seconds given the airspeed and heading.  Any difference must be attributable to winds aloft.  Does this make sense?

Yes, you are correct. You need three pieces of information:

airspeed
true compass heading (considering magnetic variation)
multiple absolute position fixes (eg. from gps)

The aircraft heading has to be measured by a compass within the aircraft (it can't be derived from the gps track), and the gps position can be used to look up the local magnetic variation. The difference between the speed/heading and the ground track (in straight and level flight, anyway) is attributable to winds aloft.

I look forward to the day when GPS is accurate and cheap enough that we can put one in the nose and one in the tail and derive heading from the difference.

@SteveV. Magnetometers are pretty accurate - and they work even when you're not moving (e.g. during the final compass/DG checks before takeoff). You could even put several on an aircraft if you're worried about local influences.

So was this calculation inaccessible pre-GPS days? Or could you do the same calculation using an accurate gyroscope too?

@curious_cat: Not gyros but accelerometers (and also gyros to figure out where the accelerometers are pointing). They're called IMU - Inertial Measurement Units. We still use them in submarines because GPS signals can't penetrate more than around 2m of sea water.

@slebetman Are compasses useless on subs too?

I thought an aircraft would use a gyrocompass for the heading indicator.

Close! Here is a simplified example:

First we use GPS positions to determine the ground vector.  This is based on distance traveled and ground track over time.  It is a fairly straight-forward calculation based on the time to travel between two coordinates and their relative positions.
Then we calculate our flight vector.  This is based on the aircraft True Airspeed (TAS) and heading.1
Lastly we use basic geometry/vector math to calculate the difference between the two vectors.  This calculated vector is the wind direction and speed.

1 TAS is Calibrated Airspeed corrected for altitude and non-standard temperature - the speed of the aircraft relative to the airmass in which it is flying.  It is calculated using the pitot-static inputs along with the Total Air Temperature (TAT) input (if available). To get an idea of the math, take a look at this answer of mine that calculates Mach Number, which is one of the values used to calculate TAS. Also, the TAT isn't really needed for low-speed aircraft that fly below about 100 KIAS.

Note that it's basically a dynamically updated E6-B calculator. The AHRS isn't doing anything you couldn't do on a circular slide, it's just doing it faster and on the fly.


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Why in certain airports the windows are blocked while others not?
In certain airports, like the T1 of MMMX the windows are blocked or semi-blocked

while in others are not (MMGL)

I understand this could be done to avoid debris hit the windows, but then why?

Are they not simply sun shades?

@TobySpeight Its too wide and heavy to be, but I didn't think this way.

I don't see that this has much to do with aviation. Airport windows are presumably like windows in any other large building, so the considerations about building codes, sunlight, cleaning, maintenance etc. would be the same.

Keep the sun out if facing south or west, help keep the temperature down.

Or North, if in the Southern hemisphere. :) Or pretty much any direction in the tropics.

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Can you ride a storm to save fuel?
Cargo sailboats, of the old, depended on wind direction. Like these ships, planes also save time by flying with the wind. On the extreme end, could a plane use storm fronts or hurricanes by flying in and out, in front of, or down the length of it to save time or fuel?

Following the wind should obviously use less fuel, assuming it is a good enough idea to go that direction.

Not the same, but I read some stories about WWII recon planes where they road the jetstream back. Some people can tolerate 20,000 feed unpressurized, and some people can't.

google "glider cloud street", that's riding the underside of developing thunderstorms :-)
Bbeing it clear that atmospheric phenomena can be used to stay aloft please specify what kind of a/c are you thinking about

A crashed plane burns no fuel at all.

What size aircraft are we talking about? The FAA recommends staying at least 20 miles from any thunderstorm activity for GA aircraft. I can only imagine that they would double, triple, or quadruple that for something as bad as a hurricane. Convective activity a lot weaker than that would keep most GA pilots from flying.
My CFI had an old saying when looking at weather radar. “Green takes the dirt off. Yellow takes the bugs off. Orange takes the paint off. And, pink takes the wings off.” Even that is putting a lot of faith in your aircraft and ability. Personally, I would be suspect of the green and would avoid the yellow all together.
Saying that, in the Northern Hemisphere, staying to your right of a low pressure system and your left of a high pressure system should provide you with a more favorable tailwind. But, how far right out of your way would you have to go in order to benefit from the winds of a hurricane without putting yourself and your aircraft in danger.

is that last paragraph northern or southern hemisphere advice, or both?

@TamaMcGlinn - Northern.

Not storms, but there is a concept called "Pressure Pattern Flying" where you plan routing to stay in favourable circulation around Highs and Lows, to the extent that deviations to follow the circulation flow and stay in tailwinds can get you there sooner than going straight.  To take advantage of this you need to be going fast enough and with the range to span pressure regions. Although Lindbergh talked up this sort of thing in the 30s in his own flying, it's generally something exploited by airliners on transoceanic flying.
And in fact the North Atlantic Track system adjusts the tracks northerly or southerly daily to maximize or minimize the effects of winds around pressure systems, especially focused on jetstreams, which form at frontal boundaries at the tropopause. It might be worth a significant deviation from straight line point-to-point flight to ride downstream in a 100kt 10 mile wide jet that's going your way at 31000 ft (or whatever level the tropopause is at for a given region and time of year).
As far as storm systems go, nothing good happens near one.  You stay away from them regardless of the size of the aircraft, 20 miles being the convention, and ideally on the upwind side.  One hazard of passing by a thunderstorm cell on the downwind side, even if well clear of it, is running into hail falling in the clear air after it shot out the top of the cell.

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Short Take Off and Landing, and Range
Can a plane have good range with a short take-off and landing (STOL) capability? If not, why. I have noticed STOL planes with lack the better good seat miles per gallon of planes that need longer runways. Please correct me if I am wrong.

"Good range" is so subjective. The Antonov An-72 has a range of almost 2700 nm, so that's pretty good, and it can handle pretty short runways. Different nations' authorities differ on the definition of STOL. Generally speaking, however, most STOL airplanes are going to have an unusually high lift-to-weight ratio (big, fat wing), and an unusually high power-to-weight ratio (big, thirsty engine) that's going to preclude a long range due to the trade off in drag and fuel.

I had written up an answer when someone closed the question.  I went ahead and pasted my response as comments.  I believe there is a non-opinion based answer to the question.

Aircraft design is always a series of compromises.  We sometimes work on it in terms of a constrained optimization problem.  In this form, one thing gets to be the objective (minimum cost, or minimum weight, or maximum range) and everything else is a constraint (required takeoff distance, required payload, required speed, required range, etc.).
If you think about the space of all possible solutions - just the objective, no constraints.  You have the most freedom to design.  Each requirement you add removes some of the available design space.  You have less freedom to design.  Some requirements may be benign "the aircraft must be red" - probably won't change the maximum range solution much.  Other requirements are very difficult to achieve "the aircraft must fly supersonically".
A STOL airplane is one that has been designed with more emphasis on short field performance than an otherwise similar aircraft.  Because of the emphasis on STOL, a different set of compromise decisions have been made.  Most likely, it adds some weight (high lift systems, rough terrain landing gear, a larger wing).  Some of those changes add drag (wetted area, protuberances).  The larger wing can fly slower, but it is less efficient when flying fast.  These STOL additions likely affect performance in other aspects - top speed, efficiency at speed, range, payload capacity, cost.
Technology can help -- perhaps a brand new clean-sheet design can do better than a 40 year old design.  However, comparing same-technology levels, the aircraft with more requirements placed on it will have to make more sacrifices  in other areas.

Yes, they can.  It's called retractable slats and flaps.
One plane in particular is the Fieseler Fi 156 Storch.  Legendary short take off performance.  Would have cruised at a much higher speed if it's slats were retractable. 400 ton airliners use them for a very enviable flight speed range of around 150 to near 600 mph.  Imagine if they were a bit lighter!
In reality, building a few more feet of runway removes most STOL requirements and allows the long range cruisers to use every bit of saved weight for fuel and cargo, which will add up to a lot more $$$ over time.
But there is no reason why a smaller airplane cannot have retractable slats and flaps, except that, invariably, more weight and a larger motor with more fuel requirement makes the slightly longer runway the more popular option$^1$.
$^1$ the helicopter being another

RATO is another.  These days a mini Falcon 9 underneath just might do it and land itself for another go.


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Can thermals or high density altitude improve fuel efficiency?
Suppose a pilot is just cruising around building flight time. In order to get the most flight time for fuel, would flying on a dry cold day or on a hot thermal filled day save fuel?

I'd think there's no clear answer here. Imagine a motor glider which would be able to not use it's engine at all, the answer is obviously "thermals"; if you're flying a 747 there's no way you can center in a thermal and you'd prefer a cold day. All other aircraft fall somewhere in between these extremes.

@Sanchises I did say getting now building flight time. So what ever the minimum or ideal plane to build flight time, not a airliner.

I imagine at that point you could just get a glider, duct tape an engine to it, and go for thermals. There are gliders that meet that description iirc.

Unless you shut the engine off you are still burning fuel.  Sure, maybe you could reduce the throttle in a thermal and get a little “free” altitude, but in a typical gas powered time building Cessna or Piper the benefits are probably minimal.

If the goal is simply to build time, a pilot could certainly save fuel by using thermals in a light airplane.  To get an appreciable benefit, he or she will have to actually circle in the thermals, glider-style.   He or she would benefit greatly by having a variometer along-- a variometer is sensitive rate-of-climb indicator designed to have minimal lag.  Small portable electronic versions exist, generally designed for hang gliding and paragliding usage.  One strategy would be to initially do a powered climb to near the top of the thermal convection, then reduce power to a setting that gives a slow descent and start looking for thermals to circle in to re-gain altitude without changing the power setting.  However the truth is that many pilots will find circling in thermal updrafts in a light airplane to be rather fatiguing-- it involves lots of time going around in circles in bumpy air.  It's a good way to expedite a climb to altitude, but it can be a tiring way to spend an entire flight.
Other options for saving fuel would include use of ridge lift or wave lift.  If the aircraft is based near a long ridge, opportunities to soar the ridge at reduced power settings may be relatively common during certain times of the year when the wind direction and velocity is favorable.  Using mountain wave is a more exotic proposition that may often require a long transit to the wave or a high climb to contact the wave, but I'm aware of a case where a Cessna 152 was flown in wave for over an hour with the motor at idle power (or lower) at around 6000' above ground level at a location about 15 miles from the home airport.
All things considered, the most practical way to save fuel on a regular basis is simply to operate at reduced power settings.  But it certainly is technically feasible to take advantage of various forms of updrafts as well.  This will take a concerted effort on the pilot's part-- it's not something that will happen automatically when the pilot is "just cruising around" on any given day, regardless of the prevailing meteorological conditions.
If the question is meant to be essentially "is it more efficient to fly in stable air or in convective conditions", the answer is "it depends".  If a pilot is trying to maintain a constant altitude, then the fuel consumption will generally be lower in smooth air.  If the goal is to minimize fuel consumption, the worst thing a pilot can do is to push the nose down to maintain altitude whenever the aircraft encounters an updraft.  It is better to hold the airspeed constant and let the aircraft climb, and better yet to reduce the airspeed and let the aircraft climb still more.  Then if desired, the airspeed can be increased to gradually shed altitude once the aircraft is out of the updraft.  This strategy will reduce fuel burn during cross-country cruising flight at an average airspeed that is higher than the maximum endurance airspeed.  However, if the goal is really simply to minimize the fuel consumption while accumulating hours in the air, it would be better still to simply fly at the maximum endurance airspeed in smooth air, and better still to fly at the maximum endurance airspeed on a thermally day while actually making efforts to stay in the thermals as long as possible by circling.

For flight time and saving fuel, the density is not relevant. Density is relevant to distance covered if you need a set cross country distance rather than a set time. You would only want to set power to maximum endurance which will be very near best glide CAS, while best true airspeed and ground speeds would change with density the best endurance CAS would not.
It may also be advisable to plan relatively shallow descent angles so that the engine stays within a reasonably efficient power range. Climb with power low enough that you can keep it leaned.(after the initial takeoff and climb out, unless maybe you are at an excessively long desert runway)
Thermals could aid the flight time allowing even lower throttle settings but could also harm as they go both up and down, capturing the right current depends on the pilot's glider experience.
Now where I rent, fuel is included in the per hour price and the goal is usually to get a set distance or set of maneuvers in a minimum time. Just the opposite of what you propose.


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Is it feasible to replace a C152's engine with multiple small turbine engines?
If I wanted to remove the piston engine from a cheap plane like a Cessna 152 and add small turbine engines with an equivalent thrust but weighing less, where would be the best place structurally to mount them? I'm thinking one small turbine on each side of the tail and two on the wings.

What problems may I expect?

I'm not sure if this post is just trolling, or the OP really considers this to be an option. FYI, you can't just "mount 2 turbine engines" to a 172, there is a lot of structure that needs to support it. Not only is this not possible, it isn't even within the realm of reality (FYI, the engine in the front adds a lot of "ballast"), even if you did mount 2 very small engines on it the wings aren't designed for that.  VTC as off-topic.

"Can a Cessna be coverted to a jet?" You mean, like, secretly loaded into a Lockheed C-5?

@RonBeyer I'm not. I would like to buy a plane with a bad motor and by-pass it with a more efficient external engine. I hope the picture illistrates the size of the replacement engine, You'll see they do not weigh much.

@Justintimeforfun You have a lot more issues regulatory speaking than just strapping them on and taking it up. You'll need to convert it over to experimental and do a whole lot of flight testing before the FAA will sign off on it. At that point, you're better off buying a kit aircraft with a jet engine that you can fly without as much red tape, money, and risk.

I edited your question to - hopefully! - clarify it. E.g. you said you were asking about 2 engines but then mentioned 4: 2 on the tail and 2 on the wings. If I got something wrong then of course you can edit again or just roll back my changes.

If there was ever a reason to have an "experimental" category, this is it.

A few linguistical issues set aside, there is no reason to downvote this question!

@Jpe61 Thanks. That looks much better. I am really thinking about this for redundancy, efficiency, and cost less, and may be quieter inside the cabin.

Have a look at the Cri-Cri propeller and jet versions.  Yes, jets can be mounted near the nose, but the plane must still balance at within its specified CG range.  This is why removing the engine in front and placing jets in back is out (unless you want to really go crazy and mount a Gatling in the front). Remember, it must balance.
However, at Cessna (152) speeds, props are vastly more efficient, so a $turboprop$ is not out of the question and, indeed, these are found on crop dusters.
"Pocket jets" are also found on motor gliders, and I have often thought of putting 4 of these on a sailplane.
However, another key consideration is how much stress an airframe can take.  With jets come higher speeds.  These planes are built to flying within a certain speed "envelope" regardless of propulsion source.
The "Screamin' Sasquatch" may interest you.  Here a jet is mounted on a sturdy bi-plane, and yes, it works.

Very nice!!! Will the "SS" fly without its main engine, minus the weight?

put a more aerodynamic nose on it? I removed the accepted answer to draw mor answers. If nothing comes of it, I will put it back. Thanks for helping.

@Justintimeforfun  if you removed the main engine you would have to put all four jets in front to even have a change to keep the CG in spec.  However, just like the 737, this would lead to pitch stability issues, requiring a larger tail.  They could be mounted on the wings, but this would work better for a glider that is already balanced.  If you keep the prop engine (good idea), jets on the wings would help take-off and climb greatly, but watch that Vne.  Expect much higher fuel consumption, but the jets can be shut down in cruise.  I like them as extra engines.

Yes you are right so similar to the "SS" they would have to go up front, unless you can put the fuel there. The idea is to climate the weight of the engine to make the plane more efficiant not to give it more thrust. maybe they could be mounted around the engine housing so balance is not a problem. They must make an turbine engine as effective and efficient as a piston engine?.

@Justintimeforfun your ideas have merit, unfortunately, you may find fuel consumption will be the deciding factor.  In the airspeed regime of the 152, props rule.  You will need a much larger fuel tank for your jets.  It would be cool to put 2 or 4 on the wings.  This was known in the 1950s as Jet Assisted Take Off or JATO.  They do attach a very large rotor to a turbine (helicopter), but the bigger, slower, wing is much more efficient.  Wing>rotor>prop>turbine.  See?

Yes this makes since. Turboprop. is ideal. My A&P buddy said he refuses to work on reciprocating engines.

You asked for it, you got it:
The x number of small non-certified engines will not add redundancy,  they are in fact very noisy, and terribly uneconomical (as a reply to your comment).
The good old piston engine is extremely reliable compared to these mini jets. Certified GA pisyon engines are tried and tested, and you can bet your mamas behind that they will function properly the whole service interval. The mini (or micro) jets, not so much...
The mini jets are stupid loud. It is incredible how much noise they make compared to their size. Having multiple of those sitting in front of you screaming like nothing you could ever imagine, well... I think they actually are the most efficient way to convert fuel into noise. Which brings us to:
They will also be disappointingly unefficient (other than producing noise). As Robert already commented in his answer, they would be operating in a suboptimal speedrange. Sure, they are used in hoverboards and other doohickeys that are even slower than a C-152, but the reason they are used in such applications is their size and relative safety compared to piston engines with large propellers.
All that (and Roberts excellent answer) being said, if I had the time, I would certainly build what you have proposed in your question. Just for shits and giggles

This is great. Thanks. I thought since they would be outside the fuselage they might be quieter. I still have much to learn.

The structure of C-152 (or any small aircraft) offers very little sound insulation, as they are single sheet acrylic or aluminium with plastic inner liner.

This is such an interesting question. I myself was thinking of adding triple microjets on both wings, for redundancy.
To add @Robert-DiGiovanni's good answer, the balance seems to be the tricky one. I'm thinking aloud here. Now, the plane must be able to be lift at the ends of both wings with

Full tanks, Empty Tanks and ballast
Full throttle, Empty Throttle
Full Speed, Stall Speed
All control surface positions
All flaps settings
All and the worst combinations of these.

These and without stalling any of the wings, tail, flaps and control surfaces.
In my scenario

removing the engine from the front causes nose up
adding the engines under the wings causes nose down when idling
engines cause nose up in full throttle.

Adding fuel tank to the nose instead of the engine helps little as sooner or later the nose fuel tank is empty and again the forces need to be countered.
Now, what comes to fuel considerations, adding six JetCat P80SE (http://modelaircraftcompany.com/newshop/en/home/76-jetcat-p80-se.html)  replaces the consumption with about 100 litres/hour. This is in the same ballpark as Jetranger 206B (100l) or Caravan C208 (150l). Don't expect to get much less for "cruise" instead of "peak", for turbine engines it's about the same (80-90%). For four hour endurance needs 400 litres (dm3) instead of the normal C152 70 litre tank. So the front space is needed for fuel.
As the @jpe61 put it, the noise would be incomprehensible so some kind of noise insulation around the engines is needed, adding to the drag and weight.


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Can I take position behind a plane for V formation legally?
First, I must find a plane with the same capabilities to fly in its wake. As a pilot, what would I need to locate other planes near me? Or find and weight on a plane going in the direction with equal cruising speeds to follow? Then would there be and issue of just taking position without asking. Should I ask for permission to do so, from the pilot which I'm shadowing or no I need more?

"First, I must find a plane, to fly in its wake" What are you following? A Cessna or an A320? Wake turbulence can be very hard on aircraft, and has caused crashes and these incidents/crashes. What airspace are you flying in? What type of equipment do you have on board? How close do you want to be to this aircraft?

@RonBeyer I'm just looking in the legality of it and the equipment or knowledge needed to do so for 2 planes of similar capability.

The "legality" changes based on airspace and how close you want to be... If you are flying in Class A airspace, you need to coordinate this with a controller and they won't break standards to get you in the wake. Same with other controlled airspace. This is different for uncontrolled, and you may be able to get as close as 500 feet, but I wouldn't do that without coordinating with the other pilot.

@RonBeyer thanks. The controller can say no, also the person who would help pair two or more planes for this? Even if the other pilot may be ok with flying together to save each other fuel?

@Justintimeforfun if you're thinking that you can save a few pennies in fuel by riding in the vortex field of another airplane like a goose, forget it. A: you would have to be so close that you are basically in tight formation flying, which is quite dangerous.  B: If you are close enough to be riding in the out and up circulation of the other airplane, it will be quite bumpy and unpleasant.  C: Extended periods of more than a few minutes in such a tight formation are very tiring. You can drive 6 feet behind a semi and raise your gas mileage dramatically, but you won't want to do it for long.

@JohnK Birds are professionals

@JohnK: at one point MythBusters actually tested out fuel consumption with an acrobatic flight crew in various formations, including the V and a single-file formation (!).  IIRC, a tight V formation was relatively smooth, but the single-file formation was just awful.

@MichaelSeifert Siefert Did they actually find a fuel consumption decrease that was measurable?

@JohnK The fuel consumption in the V formation for non-lead planes was decreased by 3-5%.  Not a huge savings, but not completely negligible. For the rest of the formations, there was no savings.

What really works better I'd say is for a small airplane to surf in the upwash ahead of the LE of a large airplane.  More or less ridge soaring. When Lishman taught geese to formate on his ultralight, frequently individuals would move forward and above the LE, and were just about able to stop flapping.

Apart from any legalities there is the question of whether or not it is a good idea.  To this question the answer is a resounding no.  Have you had formation flight training?  How current are you?  What about the other pilot who you haven't briefed with, or even met?!  What if he is on an IFR clearance and while you are focused on maintaining position he flies into a wispy layer you didn't see coming and you lose sight, what are your IMC lost sight procedures?  Unbriefed formation with pilot you don't know is just a really, really bad idea.

Here are the regulations in 14 CFR 91.111:

§91.111   Operating near other aircraft.
(a) No person may operate an aircraft so close to another aircraft as
to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by
arrangement with the pilot in command of each aircraft in the
formation.
(c) No person may operate an aircraft, carrying passengers for hire,
in formation flight.

"Formation flight" is defined in the Pilot/Controller Glossary:

FORMATION FLIGHT− More than one aircraft which, by prior arrangement
between the pilots, operate as a single aircraft with regard to
navigation and position reporting. Separation between aircraft within
the formation is the responsibility of the flight leader and the
pilots of the other aircraft in the flight. This includes transition
periods when aircraft within the formation are maneuvering to attain
separation from each other to effect individual control and during
join-up and breakaway.
a. A standard formation is one in which a proximity of no more than 1
mile laterally or longitudinally and within 100 feet vertically from
the flight leader is maintained by each wingman.

And there are ATC instructions for handling formation flights but they're too long to quote here; see the ATC Orders 2-1-13.
So, let's assume that the distances between the aircraft in your proposed flight would make it a formation flight, per the FAA's definition in the PC/G. Putting all that together:
As a pilot, what would I need to locate other planes near me? Or find and [wait] on a plane going in the direction with equal cruising speeds to follow?
No idea. This depends very much on the type of aircraft and operation. Two friends in C172s flying half an hour for lunch somewhere? Two Boeing 777s flying internationally? But practically speaking, this would be near impossible without prior coordination on the ground. How would you know what route the other aircraft is following, or what cruising speed they plan to use?
Then would there be [an] issue of just taking position without asking. Should I ask for permission to do so, from the pilot which I'm shadowing or no I need more?
Per 91.111(b) you must have an arrangement with the other pilot first. In theory you could do that in the air but people who do formation flying seriously - and safely! - spend more time on planning than on flying. I don't know many pilots who would form up in formation with a stranger who just asked.
If you're thinking of using the V formation for commercial passenger flights then see this question. Although as you can see from 91.111(c), it isn't legal today, even if the pilots coordinate and work out all the necessary procedures first. And that's without even considering what their employers and insurers would say about it.


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Could a wing be designed to exploit both normal flying and ground effect?
Ground Effect planes' wings are stubby and the height over the ground or water is limited to wing span.
Could a wing be designed to exploit both normal flying and ground effect together to give the plane more clearance over waves without loosing all the benefits of the ground effect?

The biggest problem for ground effect aircraft is they are stuck too close to the ground, so they can only fly over water, or billard table flat, treeless, terrain.  A limitation of only being able to fly over water, practically speaking, kills the concept as commercial proposition.  Which is why you don't see any zooming around in the normal world.

Note that the A-90 was design to fly both in ground effect and at 10000ft

@ManuH interesting aircraft! Looks like a big compromise on both flight modes.

And you end up with a crappy airplane (low ceiling) and a crappy ekranoplan (too much span) combined, the usual result when you try to bridge configurations with those sorts of compromises.

@JohnK I don't know about that. Is it to unrealistic that a wing could not have a design to do both tasks efficiently. Take a look at the seagull capable of using both ground effect and normal flying efficiently.

It's just that in the real world you end up giving up the best attributes of the individual features when you try to combine them.  Like flying cars.  Yes it can be done, but you still end up with a crappy car that is also a crappy airplane.  I've yet to see a flying car that is good (robust enough) for any more driving than the trip to the airport.  It's only when you really need the dual configuration bad enough that you are willing to live with the crappy duality that it makes sense - like amphibious aircraft.

Ground effect is measured using the non-dimensional height parameter 'h/c', i.e. the height above ground divided by the chord length. By increasing the chord length, the height of the maximum efficiency height is raised, as the location of the h/c does not change.
Increasing the span also adds efficient lift due to aspect ratio effects, however creating too much lift is not always better, because it will move the operating altitude above the most efficient height (h/c max).
Ground effect therefore favours larger chord length to get the required ground clearance, however larger aspect ratio wings are better for atmospheric flight. An aircraft could be designed to operate in both modes, but it will always be a trade-off between efficient ground effect flight and efficient atmospheric flight, there is always going to be a compromise of one or the other or both.

Nice answer @StuartBuckingham, as you say it's a trade-off which neither wins.

@GdD Maybe I am thinking about it the wrong way. Instead of designing a plane, it is the track that needs to be designed to accommodate the plane.

What track @Justintimeforfun? Airplanes and ground effect craft don't go on tracks.

@GdD it is a hypothetical track made for ground effect planes.

The key word being hypothetical @Justintimeforfun


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Which aircraft has the best fuel mileage per person?
I was looking at a BD-5 that got 32 miles to the gallon for one person and wonder: Which plane gets the most fuel mileage if you divide the number of passengers against the mileage of the plane? Blimps, if flown decisively, can get the most drifting, but for this question I want to narrow the questions to planes and excluding operational costs other than fuel.

Jumbo jets rule the "ton-miles/gallon" category because they can go high, taking advantage of TAS - IAS difference in thin air.  But they are only 1/4 as good as tractor trailers, albeit 10x faster.

Not a duplicate, but related discussion: https://aviation.stackexchange.com/q/88709/7532

There are a number of considerations in answering this question. This will emphasize how aircraft are actually used rather than some best case scenario, as a giant all economy flight might be. The short answer is 787-9 overall with the 321neo making an unexpectedly strong showing.
Widebodies as a class had these characteristics in 2019 as determined by an MIT study: 1.53 departures per day, 265 seats per departure, 12.49 hours airborne per day, 1932 gallons of fuel per hour, and 4,152 nm stage length. Multiplying this out you get 405 seats flying 6352nm using 24,130 gallons of fuel, which is 2.5m seat miles for 24k gallons of fuel or 106.7 seat miles per gallon on average. The 787-9 is arguably the most fuel efficient of widebodies and flies each seat 50km per liter or 117 seat mpg. The giant a380 has older tech and moves each seat 100km per 3 liters of fuel, or 78 seat mpg.
Narrowbody data is more prevalent. I found this which shows more detail:

As expected the class is worse than the widebodies at 80 seat mpg. However, in one case the 321neo at 123 seat mpg (the best of this bunch) beats the 787 average by six seat mpg, but only by considering its best year against all the 787 years. The early years of any aircraft operation generally improve as efficiencies are found, but this is more than one would expect and is likely due to stage length and other operating differences.

You just gotta feel sorry for poor little MAX 8, seemed to be on par with the neo and then something unfortunate happened, so it's now way behind them.

@TooTea Aside from the obvious 'not in service' period, I expect much of the issue is stage length as Pilothead mentioned. I've noticed that the A321neo is being used on a lot more long routes that were previously using larger aircraft like the 757 and even 767 or 787. These longer stages greatly improve seat mpg as climbs to altitude consume much more fuel than cruise.

A self launching glider like this probably can save lots of energy if it flies the path where it can use its glider capabilities at least time to time. It otherwise has 100 km range done with zero fuel anyway as this aircraft is electric.

Well, to be fair we should maybe consider the 30kg batteries as "fuel" :)

Fuel may be needed to charge the battery on the ground but in Swiss mountains or the like the closest power station will likely be hydro

Also, don't forget that you could launch a glider without any engine at all from an all-electric winch (which are quite common in the EU) and have some genuinely zero-emissions flight fun. It's not a terribly reliable form of cross-country transport, however...


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Where to find generic weight take off charts for planes?
Where can I find a generic weight take off chart for planes? I looked online but maybe I'm not looking in the right place?
Related: How do I use the takeoff and landing distance charts in the C172 POH?

@757toga Maybe a chart generator with inputs for airfoil and weight along with other specs.

By generic do you want something that just looks the part? How do you intend to use one?

Such a thing does not exist. There is nothing like a "generic" aircraft, the sheer number of variables and breadth of their values makes any attempt to produce such a thing highly implausible.
Useful takeoff performance data is only available for fully engineered and prototyped aircraft. The prototype can be computationally simulated but that is not some web app, it is a team of researchers with access to specialized facilities. (Something like a flight simulator is not an engineering tool. Flight simulators just copy existing aircraft data, they do not do any fluid dynamics or similar computation.)


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What is the 0-60mph time of an aircraft carrier catapult?
Tesla's new Roadster is giving a 0-60mph time of 1.9 seconds. Truly ludicrous acceleration.
I was wondering how a fighter jet released from a steam catapult compares.

According to Wikipedia https://en.wikipedia.org/wiki/Aircraft_catapult an American carrier type Kitty Hawk has a catapult capable of accelerating a mass of 35000 kg to 139 knots (71,51 m/s), and the system's stroke is 76 m.
From the terminal speed and the stroke, a simple calculation gives a mean acceleration of 33,64 m/s/s. Hence, and assuming the acceleration constant along the stroke, the time needed to reach 60 mph = 26,81 m/s is
t=v/a => t=26,81/33.64 => t=0,796 seconds...

Ha.  Beat that, Elon.

Thanks - I take it back. The Tesla's quick but the catapult is ludicrous.

In some fighters, there are special handrests that the pilots need to grab during a carrier launch, because there is a danger that they knock themselves out if they don't have their hands holding on to something. Teslas have a steering wheel you can hold on to, but I'm pretty sure that one is used for actual steering. So, there's still some way to go. (I'm also pretty sure that the new electromagnetic catapults on the Gerald Ford class are even more ludicrous.)

Nice answer. From my calcs, that's a lazy acceleration of 1.45g for the car, compared to 3.43g for the aircraft, almost 2.4 times as much.

That's pretty incredible acceleration. On top of that, I believe they are uniquely tuned for each takeoff, in order to cater to different aircraft types and armament configurations. Fingers crossed Elon (or someone else) can find a practical use for fighter jet level acceleration to the joe schmoes of the world.

" The Tesla's quick but the catapult is ludicrous. "  And the Tesla won't even fly at the end of the launch.  More of a plummet into the sea ...

@Jorg, the cat shot isn’t that dramatic to where you’d get knocked out. Very similar to the roller coasters that linearly accelerate the cars rather then drag them up a hill. There is a cat grip that the pilot grabs along with the Power Control Lever to avoid pulling the power to idle during a shot.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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How are individual rotor/stator blades identified during engine inspection for the CFM56?
During the inspection of fan, compressor and turbine blades, what is the method of identification?
For example during borescope inspection, what is the mode of identification of stator and rotor stages? Is there any form of labeling on these blades?

I don't know specifically for a CFM56, but in general: fan blades are usually identified by a dimple on the spinner (blade #1 is next to the dimple and number from there). High pressure compressor blades can be identified by special "locking" blades, which will look different from the others. They will have screws in them to "lock" them into place.  These line up with a feature on the rotor, so they are always in the same place.  Locking blade will be #1 and number from there. I'm not sure how high pressure turbine blades are identified.  LPT blades are usually identified by finding the #1 fan blade and rotating it down to bottom dead center.  Then the LPT blade at bottom dead center is considered blade #1.

Related: CFM56 - Borescope inspection Page labeled 14 in the document explains this for a CFM56-7B. Next page shows the index hole.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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What happens if one or more rotors are stopped during autorotation on a quadrotor?
I get the basics of autorotation when it comes to a helicopter in unpowered flight, but it's a bit unclear to me when it comes to quadrotors.
My objective is to cover some directed horizontal distance while in freefall and resume control before crashing. I want to do this to save battery, rather than starting controlled flight from the start. If I get horizontally closer to a target location without using much any battery, this is optimal.
Let's say the quad has stable attitude and is in free fall. Then I brake one motor, causing the prop to stop. A naive freebody diagram says the quad will pitch since there's no longer any lift on that side.
My question is what happens after that. Does it reach some equilibrium position? Does the decreased upward lift yield horizontal acceleration relative to the ground, or will it just spin in a circle? Do the rotation directions of the remaining props (2CW/1CCW or 2CCW/1CW) affect what happens next? What if I cut off 2 motors, either next to each other or across from each other?
Thank you.

You may want to follow this article and find the original study (if this is true). Algorithm lets quadcopters keep flying on three or less propellers. It's claimed it can fly with only one rotor working. Must be clever and use inertia / gyroscopic torque.

There are a few videos on youtube of a quad going into autorotation, but I feel like my current motors are too stiff anyway. I'm mainly interested in the theory, though.

http://hackaday.com/2014/02/06/free-falling-quadcopter-experiments-end-with-splat/ In the video at about 1:00, the quad very clearly descends slower than freefall. This is (lower than) the height I'm interested in starting freefall from. But I see I may be mixing up autogyro and autorotation. As far as shutting off the motor, though, if it starts tumbling but is moving even slightly horizontally in the direction I want, that's fine too.

Thanks. Yes basically the autorotation is like a descending parachute while the autogyro rotor creates lift by converting the horizontal energy from the aft engine like a glider. Re the loss of an engine, the problem is that you need to transfer the 3-axis steering rotations onto 3 rotors, which is possible, but needs a change in the control software (possibly taking advantage of the freewheeling lost rotor). Finally the article for using only 1 or 2 rotors is here. I'm cleaning some comments above.

When I brake the one motor, I don't intend on controlling the other 3. Those are still auto-gyro/rotating/doing whatever. I want as much horizontal distance as possible during free fall before I turn all 4 motors on and begin powered flight. This is my plan to save battery, versus falling straight down or beginning controlled flight from the start. I'll edit the question to include that.

It seems to me that autorotation is going to be hard to control without being able to adjust blade pitch. That's how a helo converts the vertical motion into translational movement. A quad uses a completely different method of directional control that requires power to work. I think tipping it without power will mostly cause it to fall faster since it will reduce the airflow through the props

@TomMcW falling faster is fine if I can trade that for directed horizontal movement. Before being anywhere near the ground, I would switch the quad's controller to normal mode (vs freefall mode), throttle up all 4 motors, stabilize, and then land slowly.

@TomMcW: From the video it looks like the prop spins in the opposite direction during free fall. It looks like what marginal control there is is achieved by selectively adding resistance to the motors (can be done if you have control of the ESC software, would be very-very hard with regular PPM signals)

@MSGhero: Most of the successful autorotation by quads I've seen are done by quads with variable pitch rotors and fixed-speed, governor controlled motor (usually a single central motor). Most homebrew designs are based around heli tail rotors but I've seen a few designs where the rotors are purpose made for the quads.

@slebetman the props wouldn't spin in opposite direction unless the quad flipped. That's just an artifact of frame rate.

@TomMcW: Why wouldn't it spin the opposite direction? If you blow air from behind (as in, it's falling) it surely would. I just tried it with my prop. Try it.

@TomMcW: Also, if the quad flipped then the prop SHOULD spin in the same direction. Not the opposite direction.

@slebetman I've got brain damage. It will spin in the opposite direction. Sorry

After watching the video the problem he was having was in keeping it horizontal. It appears that the software isn't able to control the unpowered motors.
I know absolutely nothing about quads but I'm assuming with a fixed rotor pitch they must use variable voltage to the motor to control everything such as translational movement. I'm also assuming they have an accelerometer to determine pitch. The first question is how accurate the accelerometer is at low g's.
Second, when in free fall if a rheostat that is controlled by the computer could be connected across the sides of the motor then your problem could be solved. The freewheeling motors will create a voltage just like a generator so a rheostat would be able to control the current and thus the mechanical resistance to the motor. The rheostat will have an inverse relationship with the mechanical resistance (more electrical resistance less mechanical resistance). How linear that relationship is you'll have to experiment with. If you have the software switch to controlling that resistance it should be able to direct the freefall in the same way it does everything under power, only it will be more limited In its authority. If you do it right you might even be able to use the power generated to recover a little bit of the energy from the fall to charge the batteries like a hybrid car does.
Like I said, I know absolutely zero about quadecopters so I can't fill in any details, but it seems feasible.

I don't think it's ever been documented before, so I wasn't expecting a solution. However, this definitely puts me on the right track. Thank you @mins


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Why does wake turbulence sink?
There has been a remarkable story this week of a Challenger 604 business jet being rolled after encountering the wake turbulence of an A380.

A Challenger 604 at FL340 operating from Male-Abu Dhabi passed an A380 opposite direction at FL350, one thousand feet above, about 630nm southeast of Muscat, Oman, over the Arabian Sea.
A short time later (1-2 minutes) the aircraft encountered wake turbulence sending the aircraft into an uncontrolled roll, turning the aircraft around at least 3 times (possibly even 5 times), both engines flamed out, the aircraft lost about 10,000 feet until the crew was able to recover the aircraft, restart the engines and divert to Muscat. The aircraft received damage beyond repair due to the G-forces, and was written off.

I subsequently discovered that wake turbulence sinks; according to Wikipedia:

At altitude, vortices sink at a rate of 90 to 150 metres per minute and stabilize about 150 to 270 metres below the flight level of the generating aircraft.

Well, now I know where not to fly my business jet...
Incorrect assumption
(See accepted answer below)
Presumably the vortices sink because they are denser than the air they were whipped up out of, but it's not clear to me why they are denser.

Possible duplicate of How does an aircraft form wake turbulence?

Whether or not a duplicate question, the answer to this Q is contained in that answer. Essentially, the generation of lift "forces" the air downward.  This downward momentum is what causes the vorticies (i.e. wake turbulence). The vortices sink with the downward deflected air that caused them.  Eventually, like all things "in motion", this downward moving air comes to a stop.

Having an answer contained somewhere in the answer to another question does not make the question a duplicate...

Aircraft goes up by pushing air down.

Lift is created by accelerating air downwards. The same goes for wake turbulence: The vortices form at the boundary between the downward-moving strip of air (wake) from surrounding air that tries to fill the void left by the wake.
Friction will eventually dissipate the kinetic energy of the wake, but in case of heavy aircraft the motion will continue for several minutes. By that time it will sink down hundreds of feet or hit the ground. This is driven by inertia; density differences do not exist unless the atmospheric lapse rate is above the adiabatic rate and the wake warms less quickly from increasing atmospheric pressure than the surrounding air.
In the end, the weight of any heavier-than-air vehicle is supported by an area of higher pressure on the ground. Some mechanism has to transfer the weight force from the flying aircraft to the ground, and that is the downward-moving wake. When it slowly dissipates, it will leave a slightly higher air pressure below it. When that happens at supersonic speed, the more focussed pressure change is high enough to be audible on the ground.

@PeterKampf's answer is (as usual) perfectly fine in and of itself.
I would add another way to look at it, more focused on the vortices you were interested in. One way of looking at the airflow is to imagine it is simply made of two opposite vortices.
Here is how it starts: the downash created by the wing provokes the creation of two wingtip vortices, easy to see in this famous image:
Now you can clearly see the vortices are:

Opposite (left is clockwise, right is anticlockwise)
Of the same strength (they hold the same quantity of rotational energy from the moving air) by symmetry
And you may imagine they are have infinite reach (though they are stronger at their respective center, and weaker far away until their effect vanishes).

Now the key is that they both interact with each other (vertices are mathematically additive):

The left one imposes an (almost) uniform downward movement of all the air far to its right (including the right wortex)
And vice-versa, the right vortex imposes an (almost) uniform downward movement of all the air far to its left (including the left wortex).

Of course both impose a downward movement to the air in between (that's the downwash itself).
The combined effect is at T = T+1, each vortex will have translated the other some distance down. They will then be in the same configuration again (two opposite vortices, side by side and of the same strength), and so this keeps on going until they interact with the ground or dissipate into friction (viscosity).

This is just an other way of looking at the same thing as a vortex persistence is still the consequence of inertia. Also this schematic may help visualize flow speeds induced by each vortex (remember both speed flows are additive and must be superimposed:

Edit: It is all based on the lifting line theory, particularly the horseshoe vortex. I've found few good videos of the swirl interactions. Here is one.
Since the two vortices are a full wingspan away, you can consider flow induced by the left vortex in the vincinity of the right vortex to be uniformly down. Speed is orthogonal to the offset to the center, so speed is straight down. That's not the case everywhere: above the vortex, speed is inwards. Below, its outwards. Speed beyond the wingspan is upwards. But those speeds in those regions are of no interest to the opposite vortex.
By the way, smokers can generate vortex rings. Those are the same phenomenon, just with cylindrical symmetry instead of planar symmetry. Or if you don't smoke, you can do those in a swimming pool (Yes, it's violin time!)

My intuition tells me that in imposing a movement on the other, each vortex would experience an opposite movement. If the left vortex's rotation tends to push the right vortex down, it would also have the effect of climbing up it. Why is this not the case?

Because it is simultaneous, symmetrical effect. They stay side by side: before the right one can be brought around and tucked under the left one, the left also was brought down simultaneously, so they are still level. You must picture that, while a vortex generates a rotational motion globally, we're more interested in local impacts, far from it's center. Far enough that it's generally quite uniform. I'll try to find a video of the effect.


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Could flaps be raised upward to serve as spoilers / lift dumpers?
I should preface that I'm by no means an aviation person. I'm just in a aircraft knowledge obsession phase and daydreaming about designing my own ultralight (will never happen).
The idea would be to simplify and lighten a small aircraft using the flaps as spoilers. As a practical example, the same handle could be used in one movement to get from flaps raised just before touchdown to spoiler raised just after it, making it fast and intuitive.
Basically you would have on a single axis :
high lift + high drag <-> normal lift + low drag <-> low lift + high drag
Is that possible? I guess not because I couldn't find anything like that, but why?
I name them flapoilers.

The Fairey-Youngman flaps seem to do that but all I could find as explanation is "so that the aircraft could be dived vertically without needing excessive trim changes" and I'm not sure how to understand that.

Welcome to Stack Exchange! I think this is a very interesting question.

Thank you for the welcome! This forum has tons of really great questions and information, it's a great source of brain food.

"simplify and lighten": the latter will certainly be true when combining functions, the former (probably) not. Recently came across an article by one of Airbus' heads of R&D, detailing their/his vision on multifunctional wing movables. Interesting read: https://issuu.com/maurits11/docs/00_magazine_/10.

Pretty sure this is done on some model gliders but I don't have my copy of Simon's book handy atm.

Gliders use flaps with negative angle to decrease drag at certain airspeeds.  see the answer here:  https://aviation.stackexchange.com/questions/37734/do-negative-flaps-increase-glide-ratio

One reason is that any device you want to use as a lift dumper needs to be able to be instantly retracted and this is really difficult to do with a flap.  On an airliner when the lift dumpers come up, if you slam the thrust back up to TO, they immediately come back down.  A flap system with its slow moving drive line can't do this, and it's enough that it has to be retracted from landing to takeoff setting for a balked landing like that.
Plus, even in a normal landing, you would be landing with the flaps at the landing setting, and when you touch down they would need 10-20 seconds a typical drive system would need to move up past 0 to a fully up position.  You'd be stopped by then, and deciding to take off again on the rollout is out of the question.
So you can see that any system like this would require flaps that can move up and down rapidly like ailerons.  A really difficult engineering problem.
You might do this on a small airplane with manual flaps operated by a lever, but now you have to cater to twice the movement range as regular flaps, and you'd lose the mechanical leverage you'd need for manual flaps to work unless you made the lever move twice as far (by say having it move in a 180 degree arc - but now the lever handle is in the back seat).
Lots of gliders use flaps - without any spoilers - and they can in most cases be "reflexed" up a small amount, but this feature is done only to unload the trailing edge region for high speed running which has an effect similar to reducing wing area  and pitching moment to reduce both induced drag and trim drag.
For approach control and landing, they actually do the opposite of what you would think; you drop the flaps down to near 90 degrees when you want to come down steeply and for the landing roll.  Not everybody likes that kind of system because you have the same problem - you can't bring them up and down quickly like spoilers so you can't work the flap lever like a throttle on approach the way you normally do with regular spoilers/dive brakes, and they require different flying techniques, on which a pilot used to spoilers needs to be carefully briefed to avoid trouble.

I was thinking for small planes using levers yes. So it's doable as long as the movement range is carefully designed?
The technique of "flaps raised before touchdown and flaps down and spoiler up for hard brake" seem to be used by STOL bush plane flyers. They often don't have spoilers though...

Sure if you had simple plain flaps on a light airplane it would be a simple matter to set the "up" stop at say 20 deg reflexed up if you could set up the required travel range in the lever.  But I'm pretty sure you wouldn't find them all that useful in the overall scheme of things, which is why you don't see them very much.

Regarding the first paragraph... what prevents one from using fast-moving actuators for the flaps as well as the spoilers?

A fast acting actuator like one for a control surface would have to be very large because the loads are so high.  You commonly see ballscrews or rotary planetary gear actuators, and these have huge gear reductions to keep the actuating motors reasonably small and minimize the size of drive shafts.  If you want to run the flaps up and down with direct acting hydraulic actuators, (a) they have to be very large, and (b) you have to synchronize them, which introduces other design hassles.  You have to have a fail safe method of keeping a panel from retracting from air loads if an actuator fails.

@JohnK: Deployed spoilers are, like deployed flaps, subject to very high aerodynamic loads; like deployed flaps, can easily result in a catastrophic loss of control if they retract asymmetrically as the result of an actuator failure; and, in many aircraft (including virtually all non-small aircraft), are a (or the) primary lateral control surface (thus demanding even better reliability than flaps - an airplane can land without flaps if need be, although the tyres and the runway's EMAS bed might be unhappy afterwards, but landing an aircraft without lateral control is somewhat difficult); 1/2

2/2 and yet they use fast-acting actuators...

No.  Just about all airplanes have enough control authority to tolerate a single spoiler panel that floats up, or even driven extended.  And the loads are nothing like flaps.  Nothing at all.  In any case, an unrestrained flap panel retracting under air loads is a potentially Catastrophic event.  The drive system has to have enough redundancy in the drive system, brakes and such, to render the risk of a single point of failure below one in a billion  A floating flight spoiler, spoileron, or lift dumper is not.  That type of failure will be Major or at worst, Hazardous.

What is created by deflecting a flap (or aileron) up is a reflexed airfoil, which, at higher angle of attack actually might create LESS drag and contribute a lower pitching up tendency.  This design is used on flying wings, while others prefer to have pitch control duties handled by a separate tail.
Conceptually, "flapoilers" are a good idea, but aerodynamics requires spoilers to be placed further forward on the upper wing surface to be effective.
There for, they are usually a separate control surface from flaps, although a "clamshell" arrangement is seen as a speed brake.  Model gliders, as AEhere mentioned, put both ailerons up and (closer to fuselage) flaps down in the "crow" configuration to achieve spot landings in competition.
But keep spoilers in mind as the (old and new B-52) makes very effective use of them as
"spoilerons" to bank and turn the aircraft.
But, with exception of some gliders, most aircraft want to land at as low an airspeed with as much drag as possible. This favors the flap.

For radio- controlled gliders, it is common to deflect both (outboard) ailerons upward while also deflecting both (inboard) flaps downward.  This combination is called "crow", and it makes lots of drag to allow for a steeper glide path for easier landings, with no increase in stall speed.
Something similar would surely be possible with some full-scale aircraft, though as other answers have noted, there may be some engineering challenges.
Actually what you are trying to do is INCREASE the stall speed when the surface is deployed.  (So after the wheels touch, the flaps suddenly snap to the raised-upward position.)    My point is just that in the R.C. world it is possible for a control surface to be fast-moving AND to hold position in the face of heavy aerodynamic loading, so there must be SOME way scale this up.  Whether that would ever be considered an optimal or good approach, is another matter.

( I now see some other answers have specifically mentioned crow)

Current designs do not support that.  In planes with Fowler style flaps for example, the flaps extend down and back on a curved track.  The top back of the wing extends over the top of the flap when retracted to make a smooth surface.  The track would have to be redesigned to allow the flap to go up.
I have not felt the need for spoilers in my 4-seat plane.  Any big altitude losses, generally when coming in too high on final, can be accomplished by side slipping the plane - lots of right rudder while banking left with the ailerons for example. Makes the plane nice and draggy to enhance vertical descent rate.

The concept you're sketching would reduce effective camber and increase pressure on upper wing surface (both of which is desired in a spoiler). It would however also produce a "nose up" moment while moving into position, relatively strongly so just before the flow on the underside of the flap separates -- and that's not really desirable.
The classic spoiler sits further forward and starts separating flow the moment it starts being deployed. Because it sits further forward, a larger portion of the flow on the upper surface is affected, and because usually the flap is deployed (downwards) at the same time, that makes two surfaces which will create significant drag, but not a lot of pitching moment.
...oh, and if you used the flaps as spoilers, you couldn't use them as flaps at the same time :) Except if you're flying a B2, which I believe has split flaps/ailerons, so they can move one half up and one half down -- but that's mainly due to the particular flight mechanics of a "flying wing" configuration with no tailplane, and is both less effective and less efficient than Fowler flap plus spoiler.
That said: If memory serves right, then the Airbus A350 actually can in fact move its flaps up a little from neutral position, and also move its spoilers down a bit, too. The latter is to adjust the gap between the main element and the flap, depending on flight condition, and the former to support the ailerons, adjust span-wise loading and to adjust the effective wing camber for different phases of the flight (since the aircraft becomes lighter while burning fuel, so the lift requirement changes).

That makes sense, didn't thought about the nose up moment.

Probably not since spoilers are located further up the wing (leading edge) to "spoil the air" that is moving faster over the top of the wing rather than after it falls off the back of the wing hitting the air from underneath. None the less, the idea sounds logical. My glider has spoilers do to the huge amount lift the wings produce and can send you rising past 15K feet before you know it and needing O2!! The spoilers are a separate lever and lift the flaps that are centered over the middle area of the wings and give equal "spoiling"  allowing the aircraft to slow down evenly without tipping the nose up or down like control surfaces on the trailing edges do.

Supposedly, you can do this on the Cessna 195. The claim was that by moving both flaps upwards a very slight amount at cruise, trim drag was reduced.

Why upwards? They don’t dump much lift at the back of the wing. They do increase drag and reduce wing area a bit, like when deflected fully downwards, where they are much closer to after touchdown.
Re the design in Peter Kãmpf’s answer


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Do aircraft always have the right of way on the ground? Or must they yield to police/fire/ambulance?
I know FAA said aircraft always have the right of way over ground vehicles. What if a police car, ambulance or fire truck needed to pass?

Just because one has the right of way, does not mean they must exercise that right.
If a pilot sees an emergency vehicle, and is able to, I expect he would stop, and either flash his lights or wave visibly to allow an emergency vehicle to proceed.
The aircraft has right of way, however, because the pilot may not see an emergency vehicle, or may not be able to stop quickly and safely.
So, yes, the aircraft always has right-of-way over ground traffic, according to the rules.  They are reasonably expected to yield in certain circumstances.

Of course, at a controlled airfield ATC is likely to tell an aircraft to stop if there is some other emergency going on


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Where are the first two waypoints for runway 4R departures from KBOS?
Departure way points for Runway 4R at Logan Airport in Boston Massachusetts. Can anyone send me a map depicting where the first 2 way-points are for 4R Departures?

This is going to be dependent upon the specific departure chosen or assigned by ATC.  Which departure are you flying?

@CarloFelicione All departures from 4R have NHANT and HURBE as the first two waypoints.

Checking the IFR departure plates for KBOS, the first two waypoints for all departures from Runway 4R are NHANT and HURBE. You can see where they are on the Google map below.
BLZZR THREE departure plate for reference.

the first two waypoints for all departures from Runway 4R are NHANT and HURBE <— This is not strictly true. This does appear to be true for all RNAV based departures with departures specifically listed for 4R (those with waypoints) but is not true for the Logan Two which calls for headings to DME distance.


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How can a Boeing 747 take off from Princess Juliana 7,546 ft runway?
I recently read this article that talks about 747s taking off from Princess Juliana International Airport in Sint Maarten.
The Princess Juliana runway is only 7,546 ft, but according to Wikipedia, the 747 needs more than 10,000 ft of runway for take-off.

How can the 747 take off from Princess Juliana?

Accelerate in ground effect over the ocean?

Corsair has (had?) scheduled flights to TNCM, with B747-400. Youtube.

Landing and takeoff distance depends on weight, winds, temperature and runway surface. Boeing for a while flew 747s into Renton's 5000 foot runway.

I guess that is for a safe takeoff, e.g. if the pilot want to give up he/she can stop within the runway. The actual lift off distance should be much shorter.

A completely empty 747 Dreamlifter took of from a 6100 ft runway, after some Atlas pilots mistakenly landed at the wrong airport - https://en.wikipedia.org/wiki/Boeing_Dreamlifter#Incidents

You would have to go the the manufacturer approved performance charts for this as opposed to Wikipedia as I don't really know where the author obtained that.
But Boeing published the following airport planning data for the 747:

This chart would be representative of a worst case scenario on a hot summer day (90°F) and calm winds.  Even under these conditions, the airplane can still depart using a 7000 ft takeoff roll with an GTOW of 700,000 lbs.  Practically speaking, this would allow for a full passenger and cargo complement plus 200,000lbs of fuel, which should enable it to fly 3000+ miles with IFR reserves, clearly enough to reach a suitable airport in the Americas, Europe or Africa.
And it really does work, albeit a little hairraising for the tourists on Maho Beach!

This guy is rotates and gets airborne just shy of the aimpoint markers for runway 9, giving him about a 6000 ft takeoff roll.  Ambient conditions and GTOW of this particular flight are unknown.


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What is the meaning of this information provided by Air Traffic Control?
Air traffic controllers seem to broadcast this information, is anyone able to tell me what this is or what its use is for?
LTBA 162120Z 03008KT 360V080 9999 BKN035 06/02 Q1028 NOSIG

Especially the last part, what does NOSIG mean?

NOSIG means no significant change since the last update. The information is called a METAR.

This is a METAR (meteorological terminal air report), a routine weather report for aerodromes. NOSIG means no significant changes are expected within 2 hours.
METAR are jointly standardized by ICAO, the international organization for civil aviation and WMO, the international weather organization. Codes used in METAR (also SPECI and TAF) can be found in the reference document from WMO: Aerodrome Reports and Forecasts: A Users’ Handbook to the Codes.
There are also decoders online.

METAR
From the Wikipedia article

METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.

The meaning of this METAR is:

Source: E6BX
NOSIG
A METAR may contain a TREND section to indicate the current weather trend. The TREND period is two hours. From Aerodrome
Reports and Forecasts: A Users’ Handbook to the Codes:

A TREND forecast consists of a concise statement of expected significant changes in the meteorological conditions at the aerodrome, to be appended to a routine or special report (METAR, SPECI). The period of validity of a TREND forecast is 2 hours from the time of the report, and this report forms an integral part of the
forecast.

The TREND forecast indicates significant changes in respect of one or more of the elements: surface wind, prevailing visibility, weather and cloud. Only those elements for which a significant change is expected are included.

When no significant change is expected to occur, this is indicated by the abbreviation NOSIG


text:
Would the benign spiral maneuver be one alternative to dealing with being "stuck above" or inside a cloud?
Recently I provided an alternate answer to a question involving "hands off" recovery techniques.  In this I briefly illustrated the benign spiral maneuver.  For completeness I will reiterate what I said in that answer:
This (benign spiral) maneuver is considered to be the safest way of recovering in a glider when one has found themselves above a hole-free cloud layer or inside a cloud.  My club teaches this maneuver as part of initial glider pilot training, though it is not (yet) part of the US FAA Glider Practical Test Standards.
The steps my club recommends are:

Full spoilers
Trim slightly aft (Other references suggest 1.5 times the stall speed)
Hands off stick
Feet off rudders

The effect of this maneuver is the glider will settle into a gentle spiral with a roughly constant (and not load-exceeding) airspeed and altitude loss.  The maneuver is also taking advantage of the aerodynamic stability mentioned in other answers which is inherent in most (all?) modern gliders.
Here are some additional sources that discuss this maneuver:

Scottish Gliding Centre's Airfield Procedures, Page 17 - 18
This post, which quotes "Transition to Gliders, A Flight Training Handbook for Power Pilots" by Thomas L. Knauff.

That got me to thinking about whether this technique would be applicable to recovery from the same situation in a powered airplane.  Being "stuck above" or even inside a cloud has certainly been the cause of accidents.  One harrowing story that really hit home for me is told in this video from the Air safety institute.
So, my question: Would a benign spiral be one alternative to dealing with being "stuck above" or inside a cloud?  Lets assume for the sake of argument that the airplane involved is a typical single-engine, forward prop GA aircraft (e.g. Cessna 172, Piper Cherokee, etc.)

Isn't the point of this manouver to allow pilots with no instrument experience and/or no instruments to descend through a cloud without getting into pilot-induced extreme attitudes? That's an extremely unlikely situation in a powered plane.

My understanding is that this does NOT work in all modern gliders.

Another video from ASI where someone found themselves is that very scenario. I don’t quite remember, but I think ATC found that guy some VMC.

@quietflyer yes...that is mentioned in the second reference I provided.  "Not all sailplanes are stable in this mode.
Sailplanes with all-flying tails or with flaps only for
glide path control, probably will not perform a safe
benign sprial.  Even different sailplanes of the same
type may react differently with different weights or
center of gravity locations."

It works in most gliders because they tend to be neutrally stable in roll and - the big one - have speed limiting spoilers/dive brakes.  I'm not sure it's safe to do that in an older glider without speed limiting spoilers.
In any case, most power planes, once disturbed beyond the ability of dihedral effect to restore level flight, will naturally descend in an ever steepening spiral without pilot input.  There aren't any ways to limit the speed on a GA power plane (gear and flaps won't cut it) the way spoilers/dive brakes on a glider can.  It would need a speed brake that can hold the plane to below Vne in a vertical dive.

In model airplanes, this is referred to as "negative spiral stability" -- the tendency for a spiral to tighten.  Back when airplanes were covered with fabric, loss of horizon reference without instruments to fall back on tended to lead to the "spiral of death" -- though careful attention to airspeed might alert a pilot, it was impossible to genuinely recover without an artificial horizon or real horizon reference.

I would rather enter a spin than a spiral dive. Even if I tried it before in clear air - turbulence inside cumulus clouds can throw the glider into all kinds of attitudes. Trying it out in clear air does not mean one is prepared for what will happen inside the cloud.

@PeterKämpf yes, if you could get the thing slowed down and stalled while still in level flight or close to it. I've heard of pilots entering spins to escape from clouds.  You'd have to take your chances that the stabilized spin won't be flat and unrecoverable when you come out the bottom.  Still preferable to coming out the cloud base in pieces.  But if you were already in a spiral, once above maneuvering speed, you're done anyway and if you're in a glider, the speed limiting spoilers is your second last option, before your parachute.

Would the brake have to be that powerful?  Can a modern glider's spoilers really keep it below Vne in a vertical dive?

Yes that's the definition of speed limiting dive brakes. You can't exceed Vne in any attitude.

I would not recommend this as a technique to penetrate a cloud layer. My answer assumes that the pilot involved in a non-instrument rated pilot. Otherwise they would just pick up an IFR clearance for the penetration.
Before beginning any penetration of a cloud deck without clearance the pilot would have be certain of their position and ascertain the lowest altitude to which they could descend without fear of CFIT. An Emergency should also be declared. Being in a sustained bank without visual references is a recipe for spatial disorientation - even for instrument rated pilots. If a penetration must be performed I would recommend doing so in a straight ahead manner by slowing the aircraft while VMC and configuring in a manner to increase your drag to minimize the time spent in IMC. Fly the aircraft in a straight, unaccelerated descent.
It is imperative that you obtain information about where the bottom of the cloud deck is in your specific area so that you can be certain that you will break out in VMC before reaching your minimum safe altitude. If you need to descend through a hole in an undercast and it is thick enough that you do not think you can do so in a straight ahead manner with the descent rate available, then I would recommend flying a descending box pattern on cardinal headings in lieu of a spiral for the reasons above. Flying cardinal headings means that your aircraft should drift with the hole you are descending through if there is any appreciable wind.
Here is a reference for you that describes the types of spatial disorientation that would be highly likely to occur using the technique you initially posited and why I recommended against it.

Hm..well the whole point of this maneuver is that, due to spatial disorientation, you won't be able to maintain a "straight ahead" flight path.  The situation also assumes that either the pilot doesn't have instrument training and thus won't be able to avoid spatial disorientation or doesn't have the instruments to avoid it.

It is virtually unthinkable that a powered plane pilot has absolutely no instrument training, even if they don't have an instrument rating. However glider pilots will often have literally no instrument flying experience.

Welcome to Stack Exchange! I think bclarkreston was primarily asking whether this one particular maneuver is a good idea or not. The alternative technique you describe may be useful information, but it doesn't seem to be an answer to the question that was asked.

@Tanner The very fist sentence in my response answered the question. Also, bclarkrston Any VFR rated pilot should have sufficient skills to fly an aircraft straight ahead (or in an unaccelerated descent) for a relatively short period of time in IMC. Piston powered single engine aircraft will not fly hands off as you described. So, the maneuver as you described it is simply not an option.

I'd think a forward slip would be the best option for a small hole because there's no turning at all, and even if you do enter the cloud, you just hold the controls in the exact same position until you break out.

@StephenS that is an interesting thought.  But isn't there a possibility in a forward slip with no reference to the horizon to lower the nose to much, thus generating to much airspeed?

@bclarkreston The hole means reference to the ground, and you can do a forward slip in a lot less space than a box turn or spiral. You should gain no airspeed at all if done correctly; the side of the plane makes an excellent speed brake.

@StephenS looking down through a hole in a cloud deck is NOT the same as having a horizon. Although you could achieve a much higher descent rate using a slip the most important factor is stability of the vestibular system for those not trained and proficient at instrument flying.

@bclarkreston https://www.skybrary.aero/index.php/Spatial_Disorientation#Somatogyral_Illusions Here is a reference for you that describes the types of spatial disorientation that would be highly likely to occur using the technique you initially posited and why I recommended against it.

I have flown aircraft where we had to discuss the possibility of spatial-d every flight and were trained in how to mitigate it. Still, I have attended the funerals of two of my comrades who succumbed to it. It is not to be trifled with. That is why I provided you the procedure above. Every private pilot student I sign off has to demonstrate proficiency in that maneuver before they will earn my sign-off for their checkride.

@MarkInMd Suggest you add the content you've posted in the comments (the reference video) to your answer itself.
So, are you suggesting that a Benign spiral would transform into a graveyard spiral?

I am not suggesting that it would, but that it could. It is certainly a higher risk maneuver that an straight descent. If the pilot were to still be IMC when they reached their minimum altitude there would be a very high probability that they would experience some amount of spatial-d when they attempted to roll out of the bank they had been holding - and that could be catastrophic.

I thought spatial disorientation was to be expected in cloud and the point of the instruments was to be able to control the airplane anyway.  Anyway there has long been a history especially in Europe of flying sailplanes in clouds with a turn rate indicator as the only gyro instrument, so the advice to avoid the benign spiral seems overblown- if you aren't touching the controls anyway, why does spatial disorientation matter?

@StephenS Also I'd question whether the idea of trying a straight-line slip to fit through a small hole in the clouds below you will really work- guess it depends on how small the hole is and how tall the vertical cloud-walls around the hole are.

@StephenS - - I seem to recall some aircraft having rather weird pitch stability characteristics during extreme sideslips.

@quietflyer I've done it in Cessnas and never noticed any instability. Forward slips are the standard technique for short field landings in planes without real speed brakes.

I may be recalling an early-model 172 slipping with 40 degrees flaps.  Probably  cautioned against in manual.

When I was a teenager, I was taught to do a benign spiral in a 2-33. While it works in some older trainers, it really doesn't work in many (most?) high performance sailplanes. Many sailplanes don't have enough stability to establish a steady benign spiral. Some, such as my ASW-15 are unstable in phugoid motion, so releasing the controls will result in ever-increasing pitch departures. When I was a CFIG, I did teach benign spirals, but I always made sure that my students understood that it damn well might not work, otherwise, they might develop a false sense of security.
These days, AHRS is dirt cheap, and it is available on many common flight computers and varios. The only solid solution is to install AHRS in our trainers, and to provide simulated instrument training. There's a good reason why private airplane pilots are required to log 3 hours of instrument training, and it's very unsettling that glider pilots are still being taught to let go and hope for the best.

The downside of course would be obvious temptation for glider pilots to intentionally continue their thermal climbs upon reaching cloud base.  In the thermal context-- as opposed to wave soaring and some ridge-soaring situations-- there's little likelihood of accidentally entering a cloud.

With full speed brakes the phygoid should be reasonably well damped. Still, I would rather spin an ASW-15 than let it fall into a spiral dive.

Yes, in some airplanes.  Gliders tend to be more unstable in roll than power planes, due to the over
banking tendency due the long wingspan.  A high sink rate provides a stabilizing effect in roll, due to roll damping, so be sure and bring the power back to idle and use full flaps. (This stabilizing effect due to sink rate due to roll damping may also be accentuated with a long wingspan, so it's hard to say definitively whether the technique would work in any given powered airplane with modest wingspan.)  I have heard an instructor advocate this technique for one light airplane; I believe it was a Cessna 172.  Carefully test it first before trying in cloud!
A better technique for descent with no working gyro instruments in some airplanes might be to use the magnetic compass, using rudder inputs only (hands off the stick or yoke), using a target heading of due magnetic south (if you are in the northern magnetic hemisphere.) At mid latitudes this can work suprisingly well in some aircraft; don't knock it till you try it.  Why it works could be the topic of another question for ASE.  It undoubtedly would not work at high magnetic latitudes, and it also might not very well at very low magnetic latitudes, i.e. near the magnetic equator.
With a working attitude indicator or even just a working turn rate indicator, there are better options than either of the above techniques for anyone with even a bare minimum of instrument training, but they still retain some value as interesting demonstations of the physics of flight.

Future edit: the first paragraph of this answer has some room for clarification.

Let's start with the air safety video.  Being familiar with the "canyon" the pilot tried to fly through (I-80 northeast Utah crossing Parleys Summit) his odds of survival were significantly low.  Sadly, he may have been better off attempting to land in Coalville on the interstate.
Flying into those conditions, against repeated advice was fatal, as would be attempting a spiral descent in that type of terrain (the Wasatch mountains).
For a powered plane, a 180 degree turn, or any turn, away from clouds, along with pre-trip weather awareness, keeps it safe.  What happened in the video was slightly different than a thermal chasing glider getting pulled into a fair weather cumulus cloud. We are also taught to stay below clouds flying VFR.
In a powered plane, if one is crazy enough to get into that situation, but sane enough to save themselves, number 1 (after carb heat ON) would be a slow turn away from the mountains, then fly straight and level as high as I could away from the weather making sure, with my rudimentary instrument training, to fly straight and hold altitude after completing the turn.  Then, get on the radio, ask for help, and get a vector to safety.
The deceased pilot did not follow instructions.

The benefit of using the benign spiral as a means of descent through cloud is to overcome the tendency of pilots with either extremely low instrument flying experience, or no instruments, to make extraneous control inputs based on erroneous sensations. This can result in the pilot believing they are in a normal flight attitude when in fact they are in an extreme attitude. The hands-off stable approach of the benign spiral minimizes this problem.
The situation is not very relevant to power plane pilots. Even the most basic power licenses teach some instrument flying, enough to be able to make a controlled descent through cloud without leaving the normal flight envelope. Glider pilots often have literally zero hours of instrument flying (and sometimes no attitude instruments), and are much more susceptible to the above problem. A powered pilot is probably better off remembering their instrument flying training and making a controlled descent.
While powered pilots have many accidents during descent in clouds, these are more likely to be descent into terrain or other collisions. The benign spiral does nothing to prevent these accidents, and may make them worse, since you have little control over your direction of flight.
As pointed out by John K, power planes are not usually as stable as gliders in these situations. If you are not sure that your plane will stay in a benign spiral then the technique is at least as likely to result in an extreme flight attitude as an attempt to make a controlled instrument descent.
I suspect also that the benign spiral is actually taught as an exercise in controlling the aircraft rather than as a technique to be used in any but the most exceptional circumstances.

Appreciate your response, but I would point out that your last paragraph is conjecture that flies in the face of the references I've provided.  Also, though I agree that a non-instrument rated pilot would have some instrument training, it doesn't mean that pilot would be able to escape from an unusual attitude without visual reference.  I am not instrument rated, but I did three or four lessons under the hood in a simulator where the instructor put me in an unusual attitude and asked me to get out of it.  I was unable to do this initially, despite have a non-instrument rating.

I agree with you however that you should be sure your plane can do this maneuver before trying it in these conditions...indeed, one of my references suggests trying this in "your glider" in normal conditions to find out how the it behaves.  One thing I hoped to elicit with this question is a story from someone who has actually tried this maneuver in a typical GA powered plane.

@bclarkreston You are right, the last paragraph is speculation, but I don't think it conflicts with the references you have provided much. It is not necessary to be able to escape from an unusual attitude to be able to complete a descent through cloud successfully - the point is not to get into those situations anyway. And remember you shouldn't even be trying these manouvers unless you are already in desperate trouble.


text:
Is it legal to use GPS to navigate VOR Airways?
A previous question concludes that it is legal to fly an aircraft in the US with only an IFR-approved GPS/WAAS system and no other navigation equipment on an IFR flight plan. My question is about how the pilot is then allowed to navigate.
Specifically, must a pilot fly on GPS (T and Q) Routes? FAA Advisory Circular 90-108 discusses using RNAV systems on conventional routes, so it seems that a pilot could also fly on VOR airways, for instance. In fact, 90-108 specifies exactly what a pilot can use their RNAV system for:

Determine aircraft position relative to or distance from a VOR, ... DME fix, ...
Navigate to or from a VOR,  TACAN, NDB, or compass locator.
Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
Fly an arc based upon DME.

Navigating on a VOR airway involves navigating to and from VORs, which is covered by point 2. However, the list does not include navigating to a DME/radial fix off a VOR. After taking off IFR the departure controller may clear an aircraft direct to a fix on the airway e.g. "cleared direct BOONE". Could a pilot legally use only GPS/WAAS to fly to BOONE or any other VOR radial/DME point?

Can we stipulate that the discussion assumes that it is an IFR-Approved GPS in view? Clearly, the answer would change if navigating by a handheld or iPad GPS.

In fact, an airplane without RNAV capability cannot easily navigate "direct to BOONE" or any VOR radial/DME point without resorting to Dead Reckoning.

The question would benefit from clarifying that it is about IFR flight.

Whether or not you can fly a Victor airway itself using GPS, there's nothing preventing you from entering in the specific fixes and navaids that make up the airway and flying over those. Two differences: 1) You will be flying be reference to coordinates rather than radio beacons, and 2) you probably won't be allowed the full width of a Federal airway (eight miles wide or more).

It's legal if you have a suitable and acceptable RNAV system as described by FAA Rules.
Also realize that it is only RNAV capability that provides for 'direct routing' without recourse to DR - Dead Reckoning - or Radar assisted DR needed by traditional Radio Aid based navigation.
When an RNAV system navigates to an out of service VOR, it is flying a computed course from it's position vis a vis the VOR to the VOR position. This is no different from flying from any point 'A' to any point 'B'. That is the whole purpose of the Advisory Circular being quoted. A few more extracts from the AC (quoted below) should make it amply clear.

FAA AC 90-108 extracts . . .

PURPOSE. This advisory circular (AC) is intended for the following purposes:

a. Provides operational and airworthiness guidance regarding the suitability and use of Area Navigation (RNAV) systems while operating on, or transitioning to, conventional, i.e., non-RNAV, routes and procedures within the U.S. National Airspace System (NAS). This guidance material applies to two broad categories:

(1) Use of a suitable RNAV system as a Substitute Means of Navigation when a very high frequency (VHF) omni-directional range (VOR), distance measuring equipment (DME), Tactical Air Navigation (TACAN), VOR/TACAN (VORTAC), VOR/DME, non-directional radio beacon (NDB), or compass locator facility including Locator Outer Marker (LOM) and Locator Middle Marker (LMM) is out-of-service, i.e., the Navigation Aid (NAVAID) information is not available; an aircraft is not equipped with an automatic direction finder (ADF) or DME; or the installed ADF or DME on an aircraft is not operational. For example, if equipped with a suitable RNAV system, a pilot may hold over an out-of-service NDB.

(2) Use of a suitable RNAV system as an Alternate Means of Navigation when a VOR, DME, VORTAC, VOR/DME, TACAN, NDB, or compass locator facility including LOM and LMM is operational, and the respective aircraft is equipped with operational navigation equipment that is compatible with conventional NAVAIDs. For example, if equipped with a suitable RNAV system, a pilot may fly a procedure or route based on operational VOR using that RNAV system without monitoring the VOR.

b. Qualified RNAV Systems. Describes the types of RNAV systems that qualify as “suitable RNAV systems”

DEFINITIONS. For the purposes of this AC, the following definitions are provided:

b. Area Navigation (RNAV). A method of navigation which permits aircraft operation on any desired flightpath within the coverage of ground- or space-based navigation aids or within the limits of the capability of self-contained aids, or a combination of these.

g. Substitute Means of Navigation. The use of information from an RNAV system in lieu of that from out-of-service conventional NAVAIDs and/or inoperative or not-installed navigation equipment compatible with conventional NAVAIDs.

Legal is a hard word to define in aviation. It would be better to ask whether an operation was within regulations.
It depends on your type of operation, kinds of operation, and certificate requirements. If flying under 14 CFR Part 91, the pilot would specifically be concerned with the MEL of the aircraft they are flying, and Part 91.205. This leaves the the operator free to fly IFR with only a panel-mounted GPS to fulfill 91.205(d)(2)

(2) Two-way radio communication and navigation equipment suitable for the route to be flown.

Any other requirements for navigational equipment used in flight specified in FAA AC 90-108 is more in the realm of a very strong recommendation, rather than a regulation.
The AC referenced in your question borrows it’s verbiage from the AIM section 1-2-3. AIM, in its entirety, is non-regulatory.

Yes, it is legal to use a GPS to navigate Airways.  As you point out, one simply enters two VORs in the GPS flight plan, and then the GPS will guide you from one VOR to the next.
How do they join the airway? A radar controller gives you headings to fly to get you onto the airway.  If you are on an instrument flight plan, and you are flying from a major field, they almost always have radar facilities.  They give you an initial heading to fly when you take off, and then as soon as you transition from the tower frequency to the departure frequency, they tell you that you are "radar contact", and from there, they will give you a heading to fly (vectors), or they will tell you to fly directly to a VOR or more likely when you are near the field, to a GPS waypoint.  But if you filed a flight plan using VORs and airways, it is likely that they will vector you to a point where you can join the airway, or they will vector you to a VOR.
Must they first navigate to a VOR, or can they actually navigate to a nearby fix on the airway, as is conventional?  It just depends on the airspace and the way the waypoints and VORS are set up.  You may be vectored to a VOR, you may be vectored to a fix, or you may be vectored to a point where you will cross an Airway, and then be told to join the airway at that point.

Thanks for the answer! To clarify: I am familiar with taking off and hearing "cleared direct BOONE" or something similar to join an airway. I just want to know why it's legal to fly to BOONE or some other point based on a VOR radial/DME using a GPS.

It is legal because BOONE is easily identifiable using GPS. It is also aGPS waypoint based on Lat/Long coordinates. A GPS database will be filed with these positions. Also, the GPS database will be filled with the Lat/Long coordinates of other GPS waypoints that overlay other airports, radio NavAids, intersections, etc. Those coordinates are extremely accurate. As long as the GPS satellite signals are accurate, you have an accurate means of navigation without actual Nav radios. That is why it is important to verify RAIM before flying IFR using RNAV.